1. Field of the Invention
The present invention relates to a piezoelectric ceramic composition that is free of lead (Pb) and mainly contains Sr, Bi, Ti and Nb or Ta, and a piezoelectric element using the same.
2. Description of Related Art
Conventionally, as piezoelectric ceramic compositions used for piezoelectric ceramic elements such as a piezoelectric ceramic oscillator and a piezoelectric ceramic filter, ceramic compositions containing lead zirconate titanate (PZT: Pb(TixZr1-x)O3) or lead titanate (PT: PbTiO3) as a principal component have been used. (For example, see JP 5(1993)-148016 A and JP 5(1993)-139824 A.)
However, the piezoelectric ceramic compositions containing PZT or PT as a principal component contain a great amount of lead, leading to a problem that uniformity of a product declines because of the evaporation of a lead oxide during a manufacturing process. Further, many of the piezoelectric ceramic compositions containing PZT or PT have a Curie point of about 200xc2x0 C. to 300xc2x0 C. in practice. Since they become paraelectric at a temperature higher than the Curie point and lose piezoelectric properties, they are difficult to use in applications under a high temperature. Moreover, in recent years, there has been an increasing demand for the development of lead-free ceramic compositions in order to prevent damage to the environment.
Under such circumstances, as ceramics for a sensor/actuator that is free of lead and applicable even at a high temperature, piezoelectric ceramic compositions containing a bismuth layered compound such as SrBi2Nb2O9 as a principal component have become a focus of attention. (For example, see JP 11(1999)-322426 A.)
However, although it is generally desirable that a material used for a piezoelectric ceramic element, especially, a piezoelectric ceramic oscillator, have a large electromechanical coupling factor, which is an efficiency of converting an electric energy into a mechanical energy, there is a problem that the conventionally reported piezoelectric ceramic compositions containing a bismuth layered compound as a principal component do not have a sufficiently large electromechanical coupling factor. Also, when piezoelectric ceramic filters, piezoelectric ceramic oscillators or the like are produced using the piezoelectric ceramic compositions containing a bismuth layered compound as a principal component, there is a problem that the rate of change in a resonance frequency according to temperature becomes larger compared with the conventional case of using the PZT or PT.
Owing to these problems, the piezoelectric ceramic compositions containing the bismuth layered compound such as SrBi2Nb2O9 as a principal component have not yet been in practical use.
A piezoelectric ceramic composition according to the present invention includes a ceramic composition expressed by a general formula: Sr1-x-yM1xM2yBi2(Nb1-aTaa)2-zTizO9, wherein M1 is at least one element selected from the group consisting of Ca and Ba, M2 is at least one element selected from the group consisting of La, Y, Dy, Er, Yb, Pr, Nd, Sm, Eu and Gd, and x, y, z and a in the general formula respectively are in the range of 0.0xe2x89xa6x less than 0.9, 0.0 less than y less than 0.3, 0.0xe2x89xa6z less than 0.3, and 0.0xe2x89xa6axe2x89xa61.0.
Further, a piezoelectric element according to the present invention includes a piezoelectric body formed of a piezoelectric ceramic composition including a ceramic composition expressed by a general formula: Sr1-x-yM1xM2yBi2(Nb1-aTaa)2-zTizO9, wherein M1 is at least one element selected from the group consisting of Ca and Ba, M2 is at least one element selected from the group consisting of La, Y, Dy, Er, Yb, Pr, Nd, Sm, Eu and Gd, and x, y, z and a in the general formula respectively are in the range of 0.0xe2x89xa6x less than 0.9, 0.0 less than y less than 0.3, 0.0xe2x89xa6z less than 0.3, and 0.0xe2x89xa6axe2x89xa61.0.